Unit 3: Light and Optical Instruments

1. Unit 3: Light and Optical Instruments Lesson 2: Reflecting on Reflections Reflections and Image Formation J. Pulickeel January 2007

2. Recall... • The wavelength (λ)is the distance (m) between successive wave particles that are in phase • Frequency(f)is the number of cycles per second (1/s or Hz). • Period (T) is the length of time to complete one cycle (s). An example is it takes 31557600 s for the earth to orbit the sun. Its period (1 cycle) is 31557600s

3. The Electromagnetic Family • Light is part of the electromagnetic family. • You get an electromagnetic wave when you shake an electron, thus shaking the magnetic field around it. This is similar to shaking a rope (rope waves) or air (sound waves) • If you shake an electron faster, you increase its energy and its frequency

4. Electromagnetic Spectrum • Our eyes can sense or wavelength between 400 nm < λ< 700nm or frequencies of 1014 Hz . The speed of light (c) is 3.00 x 108 m/s • As the wavelength increases, the frequency decreases (λα1/f) • Some animals can see above and below this spectrum. Some cameras operate in the infrared to take pictures in dark rooms. Wavelength λ (m) Approximate Wavelength Scale Frequency (Hz) Visible Light Spectrum 700 nm 400 nm

5. Electromagnetic SpectrumWhy do things glow? • When we heat things up we are shaking the particles. Because the particles are shaking, the electrons are shaking. • Electrons are smaller, so they shake faster. If the electrons shake fast enough (at 1014 Hz), they will glow. • This is how the incandescent light bulb works

6. QuestionWhat is the wavelength of electromagnetic radiation that has frequency of 4.62 x 1014 Hz? Click to see answer Would this be visible light?

7. Non Luminous Luminous Travelling WavesLuminous vs. Non Luminous • Luminous objects produce their own energy • Non luminous objects do not produce their own energy

8. Travelling WavesReflecting on Reflections • Reflections occur when light bounces off a surface. • When a room is poorly lit you see less light bounces off the surface

9. Travelling WavesReflecting on Reflections • Not all light is reflected – some light is absorbed • Dark surface tend to absorb most of the light that strikes them • Very little light is reflected from dark clothes.

10. Travelling WavesWhen Light is Reflected • When light strikes a reflecting surface, it bounces off. • This ray is called an incident ray Reflecting surface Reflected ray Incident ray

11. i r Angle of Reflection Angle of Incident normal Calculating the Reflected Ray Reflecting surface Incident ray Reflected ray

12. Reflecting surface i r Incident ray Reflected ray Angle of Reflection Angle of Incident normal Calculating the Reflected Ray • Draw a normalline perpendicular (90°) to the plane • Measure the angle of incidence, using a protractor • Draw the angle of reflectionon the opposite side of the normal. • Draw the reflected ray.

13. Two Laws of Reflection • A Law in science is an action or condition that scientists are convinced will ALWAYS happen Laws of Reflection • The angle of reflection will ALWAYS equal the angle of incidence

14. What’s Normal? The normal is always 90° to the reflected surface!!!

15. Two Laws of Reflection • A Law in science is an action or condition that scientists are convinced will ALWAYS happen Laws of Reflection • The angle of reflection will ALWAYS equal the angle of incidence • The incident ray, the normal and the reflected ray are all in the same plane

16. Law 2: Rays on A Plane I’m getting tired of these @#$%! rays on this @#$%! plane! A Planeis an imaginary flat surface (like a piece of paper). The angle of reflection, angle of incidence and the normal will all appear on this surface Incident Snake

17. Eye can see clearly now… • You realize that an object is in front of you because light is spreading out from that object.

18. Eye can see clearly now… • You realize that an object is in front of you because light is spreading out from that object.

19. Eye can see clearly now… When light from an object is reflected off a mirror, it reflects right back to you. This makes it appear that the object is in front of us S’up yo?

20. Do you suppose that it would be easier to read from rough pages or glossy pages?Click to see answer... It is much easier to read from rough pages which provide for diffuse reflection. Glossy pages result in specular reflection and cause a glare. The reader typically sees an image of the light bulb which illuminates the page. If you think about, most magazines which use glossy pages are usually the type which people spend more time viewing pictures than they do reading articles.

21. Glare on rainy nights... • The glare on rainy nights is the result of the specular reflection of the beam of light from an oncoming car. • Normally a roadway would cause diffuse reflection due to its rough surface. But if the surface is wet, water can fill in the crevices and smooth out the surface. • Rays of light from the beam of an oncoming car hit this smooth surface, undergo specular reflection and remain concentrated in a beam. The driver perceives an annoying glare caused by this concentrated beam of reflected light.

22. Drawing the virtual Image Draw a line from the object and ⊥ to the mirror Extend the line an equal distance behind the mirror and draw the image I2 Draw a light ray from the I2 to the eyes. The ray exist on the image side. The rays originated from the I1.Draw the completed ray

23. Drawing the virtual Image with 2 mirrors I1 I1,2 I2 Multiple images are formed when two mirrors are used. Draw Images I1, I2, and I12 , then trace the rays back as illustrated.

24. The Alien Diet Problem Aliens will eat each person if they can not guess what color hat they are wearing. Each person knows they are wearing either a red or blue hat . Alan will pick first and Jacob will pick last. Alan can see all the hats in front of him, but not his own. Ivan can see Jacobs hat. Jacob can not see anyone's hat. The aliens gave the people 1 night to come up with a plan. It is possible to guarantee that at least 9 people live! Alan Betty Cam Dave Elmer Fred Georgia Hamza Ivan Jacob

25. The Alien Diet Problem The group decides that Alan will count the number of red hats. Alan is a census taker and has great experience counting things. He can see 9 hats. If the number red hats is odd, Alan will say red. If the number of red hats is even, Alan will say blue. Betty will count the number of red hats. If Alan said red, then Betty know will know Alan saw an odd number of red hats. If Betty counts an even number of red hats, then she must be wearing a red hat. If Betty counts an odd number, then she must have a blue hat. Alan counted the 4 red hats and said “I am wearing a blue hat”. Unfortunately, that was the last thing he ever said.... Alan Betty Cam Dave Elmer Fred Georgia Hamza Ivan Jacob Betty counts 4 Red Cam counts 3 Red Dave counts 3 Red Elmer counts 2 Red Fred counts 2Red Geogia counts 2 Red Hamza counts 2Red Ivan counts 1 Red